Why is it that some people float through life in a sea of tranquility while others are constantly riding an emotional roller coaster? Why do some emotionally reactive babies grow up to become calm, centered adults and others remain volatile? Questions about the origins and persistence of variation in emotional life are some of the most important questions for psychiatric and developmental science. The goal of the proposed research is to investigate variation in infants in one system that is important for emotional life-the autonomic nervous system. To that end, we will quantify spontaneous variation in activity of the two branches of the autonomic nervous system (ANS)-the parasympathetic nervous system (PNS) and the sympathetic nervous system (SNS). Activity of the PNS and SNS, which work together to maintain homeostasis is highly variable between people, thought to imbue the stimulus environment with affective meaning, and related to variation in both healthy and pathological affective experience, both healthy and pathological. Variation appears to manifest at least by early childhood but what we do not know is how much variation exists in ANS functioning during infancy, whether early ANS variation relates to other behavioral or biological phenotypes, how stable variation is across development, and what predicts its stability. The proposed research will fill these knowledge gaps. We propose to record ANS activity from infants after they complete a standardized assessment of their behavioral phenotypes and biological profiles as part of the BioBehavioral Assessment program (BBA; OD010962). We will then evaluate whether variation in established behavioral phenotypes (e.g., temperament) and biological markers (e.g., plasma cortisol levels, c-reactive protein levels; serotonin and monoamine oxidase A promoter genotypes) predict ANS activity, and whether variation is stable across the first year of life. The long term goal of the proposed research is to establish rhesus monkeys as a good model for affect development across early development so that we can subsequently identify early biomarkers of and treatments for psychopathology.